1. Field of the Invention
This invention relates of stairways or staircases between different floors or levels in a building and to a method for constructing such stairways. More particularly, this invention relates to stairways that can be curved or spiral or helical. This invention also relates the use of preassembled modular components that can be used for assembly of such a staircase.
2. Description of the Prior Art
Conventional stairs, stairways or staircases are typically assembled in place. Although the individual components can be fabricated on site, finished interior stairs can also be assembled from prefabricated parts, including stringers, treads and risers. One stringer, or stair carriage, is located on each side of a conventional staircase. These conventional stringers extend between a lower level or floor and an upper level or floor and in some cases these conventional stringers are only supported at the ends. More frequently these conventional stringers are supported by one or more vertical supports, such as walls or columns, between the ends. Conventional stringers carry the weight of the staircase and the weight of any body that is carried by the staircase. These conventional stringers are therefore load bearing members and must be sufficient to withstand the bending moments that result from the application of gravitational forces or weights intermediate the ends and between vertical supports. Although the cross sectional area of these stringers will depend upon the size of the staircase, and of course upon applicable building codes and safety margins, it is not uncommon to use 2.times.12 inch wooden planks for stringers in conventional straight stairways. Conventional stringers in turn support individual treads, the horizontal member of a step in a staircase, and risers, the vertical member extending between steps or treads, that are positioned in notches on the inside of spaced stringers.
The stringers used for straight stairways are simple straight members with notches or a stepped configuration to support the treads. Conventional circular stairways are more complex, because they employ two curved stringers that are more complicated to fabricate. For conventional circular stairways, the curved stringers carry the weight of the stairs and the weight of any body supported by the stairway and must be strong enough to withstand these resulting bending loads. However, a single wooden member of sufficient thickness cannot be formed about a radius of curvature for a circular, helical or spiral staircase. Curved wooden stringers of this type are therefore conventionally fabricated by using a number of thinner strips that can be curved or formed. The strips are then glued together to form a laminated curved load or weight bearing stringer.
One technique for fabricating these stringers in a factory is to first construct support walls with appropriate curvatures. The innermost thin stringer components are then secured along their length to the support walls in successive layers and glued in place to form the curved laminated stringers. The temporary walls are then disassembled and notches or dados for risers and treads are then cut into the laminated stringers. These long curved stringers must then be assembled in a controlled environment and transported to the building site where the final stairway is installed. This technique is relatively expensive because the temporary support walls themselves represent a capital cost as well as an additional material cost, their assembly and disassembly is a direct labor cost, and production capacity and space must also be provided. Quite often special temporary support walls must be constructed for each unique configuration. Secondary, but costly, operations, such as notching or dadoing the laminated stringers are susceptible to error or inaccuracies and add cost. Skilled labor is necessary to make compensating adjustments during assembly. Components are not easily assembled by laborers inexperienced in this craft. It is impractical to construct and assemble a circular stairway using inexperienced labor outside of a controlled shop environment. Shipping these long curved stringers is also impractical.
One approach using this same basic technique to fabricate curved stringers on the building site is however disclosed in U.S. Pat. No. 4,918,799. That patent discloses the use of upper and lower metal patterns in the form of radially extending rays in which corresponding rays are connected by vertical beams. These vertical beams serve the same function as the temporary cylindrical support walls in the factory assembly technique just described. The laminated stringer sections are secured to the vertical beams in a helical pattern and the curved laminated load bearing stringers are fabricated. The metal beams and upper and lower radial supports are then removed. This approach reduces the cost of building special temporary walls and eliminates shipping costs when this technique is used on site. However, this approach is still relatively time consuming and the quality of the final structure is dependent upon the skill of the craftsman. Curved stairways built by this method still represent a significant expense.
Another approach that can be used to construct conventional curved laminated load bearing stringers is shown in U.S. Pat. No. 5,347,774. That patent shows fabrication techniques that do not employ temporary support walls, beams or other temporary forms to fabricate the stringers. That patent discloses the use of step structures to create the desired stringer curvature. One technique disclosed in that patent uses laminated stringer starter strips that are approximately one/fourth (1/4) inch thick and can be bent by hand. The starter stringer strips are inserted into notches on the lower surfaces of the treads and screwed to the treads. The notches on the treads are formed so that the starter stringer strips are formed to the proper curvature by assembly to the successive treads. Elongated reinforcing structures are then secured to the inner surfaces of the starter stringer strips to laterally thicken and strengthen the starter stringer strips. These laminated reinforcing structures are preferably formed by sequentially gluing wooden strips to form a laminate. A second related method of fabricating laminated stringers is disclosed in U.S. Pat. No. 5,347,774. In the second method the tread-riser subassemblies are laid out and supported by temporary columns or temporary supports. The curved laminated stringers are then fabricated, one layer at a time, below the tread-riser subassemblies using the temporary columns as a form, much in the way that temporary support walls or metal beams are used in other prior art methods of assembling stringers. These laminated stringers are then pulled together with the tread assemblies each consisting of one preassembled tread and riser. In actual practice, screws, clamps and glued wedges may he necessary. However, the use of either of the methods disclosed in U.S. Pat. No. 5,347,774 means that the laminated stringers must still be built up one layer at a time, a relatively time consuming process. If these methods are used in a factory environment either the complete, bulky staircase must be shipped or the staircase must be disassembled and shipped. Even if the staircase is disassembled and shipped, the laminated stringers are still large and bulky and difficult to ship. If these assembly techniques are used at the building site to construct the staircase, the time required to laminate the stringers can interfere with other jobs at the building site.
Another prior art technique for constructing a curved staircase is disclosed in U.S. Pat. No. 4,869,034 where the stringers are fabricated from separate interfitting blocks. The blocks are shaped substantially like a parallelogram with beveled vertical ends. The blocks are strung together using a tensioned cable that is threaded through channels in each block. Cable grippers are installed in each block to prevent the cable from being withdrawn. The blocks can also be bolted together. Shear pins and dowels must be inserted between adjacent blocks to prevent relative vertical displacement between the blocks and to counteract significant shear forces. Treads are inserted into slots on the inside of two opposed blocks. Each tread extends into aligned notches in adjacent blocks on each curved stringer to help withstand shear forces.